In particular, systems comprising parallel-flow heat exchangers and cyclone separators are known from practice for the preheating, cooling and/or calcining of fine-grained materials. Such devices generally have several stages arranged one above the other, the gas stream being conducted from the bottom to the top through all of the stages, while the solid material is delivered to the individual stages in the opposite direction.
Such systems have the disadvantage that they require an enormous overall height and the degree of separation in the cyclone separator is not always satisfactory. For instance, uncontrolled flow often occurs in the cyclones, this being caused, for example, at the cyclone inlet by the inlet gas stream being overlaid by the swirling current formed in the cyclone or by the reversal of the direction of gas flow in the cone of the cyclone. Furthermore, the particles already separated at the edge of the cyclone may be scattered back into the gas inlet stream of the cyclone.
A further problem resides in the fact that, in the case of structural forms of different sizes, the centrifugal forces change, with the same inlet rates, thereby giving rise to other separating conditions.
U.S. Pat. No. 4,318,692 therefore proposed a multi-stage preheater for cement raw material, the individual stages of which each comprise a riser and an adjoining helical and/or spiral duct. The helical and/or spiral duct has a rectangular cross-section and is connected to one side of a quadrangular hopper in which the solid material and the gas are finally separated by centrifugal and inertial forces.